Elastomers differ from linear polymers because of crosslinking. The term crosslinking refers to connections between linear polymers. The reactive chemical that creates the connections between linear polymers is called a cross-linker. Many silicone elastomers are made from linear silicone polymers that contain reactive sites along their polymer chain. These reactive sites react with the cross-linker to form connections between the linear polymer chains.
The creation of connections, i.e., crosslinks, between the linear polymers converts linear polymers such as polydimethylsiloxane fluids into silicone elastomers. Elastomers have very different physical and chemical properties from linear polymers, and the properties of an elastomer depends very much on the number of crosslinks. Thus, an elastomer with a relatively small number of crosslinks will generally be very soft, and will swell significantly in the presence of a compatible solvent(s). As the number of crosslinks increase, however, the hardness of the elastomer increases, with the result that the elastomer swells to a lesser extent in the presence of solvents. A term often used to describe the number of crosslinks in an elastomer is crosslink density. Crosslink density refers to the number of crosslinks for a given length of the linear polymer.
Unlike linear polymers, it is nearly impossible to determine molecular weight for elastomers, because they are in effect one gigantic polymer with no definite beginning or end. Elastomers generally will not flow, and so one cannot measure a viscosity for such materials. In fact, the term elastomer is derived from the same root word as elastic, a reference to the phenomenon that such materials snap back when a force is applied and then released.
Silicone elastomers can be produced from linear silicone polymers by a wide variety of crosslinking reactions. In the case of a silicone bathtub caulk, for example, the crosslinking reaction occurs between reactive silanol groups (≡SiOH) and acetoxy groups (≡SiOCOCH3). For each crosslink formed, a molecule of acetic acid is released, which produces the characteristic vinegar smell as the caulk cures. The acetoxy group in such a scenario is called a leaving group, because it is converted to acetic acid which leaves , i.e., evaporates, when the crosslink is formed. While there exist many other different crosslinking schemes to prepare silicone elastomers, those silicone elastomers designed for use in personal care applications all typically use the same basic reaction, i.e., hydosilylation. Hydrosilylation is a reaction in which a vinyl group reacts with a silicon hydride in the presence of a platinum catalyst as shown below: ≡SiH+CH2═CH—R→≡Si—CH2—CH2—R
Pt Catalyst
There are many advantages for using hydrosilylation as the crosslinking reaction. It proceeds very rapidly, it requires very small amounts of a platinum catalyst, i.e., typically Karstedt's catalyst, as known in the art, and it does not involve a leaving group. But the most important reason for its popularity is that there are very few limitations in the types of materials that can be used as crosslinkers and polymers in the preparation of silicone elastomers. For example, the SiH functionality can be part of a polyorganosiloxane polymer, a silicone resin, or some other type of silicone or organosilicon composition. Similarly, the R group can be attached to a silicone, a hydrocarbon, or some other type of organic compound. This flexibility allows one skilled in the art to graft many and varied types of functional groups into the elastomer.
While siloxane-based polyamide elastomers are generally known in the art, as evidenced by U.S. Pat. No. 4,675,372 (Jun. 23, 1987), referred to hereafter as the '372 patent, it does not employ hydrosilylation as the mechanism of reaction, and the components used to form siloxane-based polyamide elastomers in the '372 patent are not the same as, or the equivalent of, the components according to the present invention. Hence, the elastomers prepared according to the '372 patent would necessarily not be the same as the elastomers prepared herein.
While the '680 patent does refer to a method of preparing crosslinked molecules, it differs from the method herein in that the '680 patent uses a triamine, i.e., a trifunctioanl amine, in place of an organic diamine, in the preparation of the vinyl functional amide and the hydride functional polyorganosiloxane in the '680 patent is ≡SiH endblocked and contains no pendant hydrogen, as in the method of this invention. These differences are significant in that when an organic vinyl functional triamide is used, the crosslinker is the hydride functional polyorganosiloxane rather than the organic amide, i.e., see Formula IV in the '680 patent. In the method according to this invention, however, the organic diamide is the crosslinker between molecular chains of the pendant hydrogen containing hydride functional polyorganosiloxane. Hence, elastomers prepared according to the '680 patent would necessarily not be the same as the elastomers prepared herein.